This invention relates to optical switches, digital amplitude modulators and digitally modulated transmitters, and is particularly concerned with improvement of the extinction ratio of such devices.
A primary function of a switch, or digital amplitude modulator, is that in its ON state it should exhibit relatively low attenuation and that in its OFF state it should exhibit relatively high attenuation. The ratio of the ON attenuation to the OFF attenuation is known as the extinction ratio, or the cross-talk isolation, of the switch. This should be as large as possible, and for some types of switch, in some applications, is found to be inconveniently low.
A similar situation exists in respect of digitally modulated optical transmitters. The bit error rate performance of such optical transmitters is degraded by a poor extinction ratio and in an optically amplified system the problem of poor extinction ratio is exacerbated by increased signal-spontaneous beat noise on the data zeros. Optical Time Domain Multiplexed (OTDM) systems, in particular, require a high transmitter extinction ratio in order to minimise coherent mixing effects in the multiplexed signal.